Method for manufacturing hot metal desulfurizing agent and apparatus for same

ABSTRACT

The present invention provides a method of desulfurizing hot metal, which method utilizes desulfurization slag resulting from a KR hot metal desulfurizing treatment as a desulfurizing agent for hot metal again to reduce hot metal desulfurization costs and the amount of slag generated, thereby solving environmental problems.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is a Continuation Application of PCT Application No.PCT/JP01/05065, filed Jun. 14, 2001, which was not published under PCTArticle 21(2) in English.

[0002] This application is based upon and claims the benefit of priorityfrom the prior Japanese Patent Application No. 2000-178321, filed Jun.14, 2000, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The present invention relates to a method of manufacturing adesulfurizing agent for hot metal and a desulfurizing agent, and inparticular, to a method of manufacturing a desulfurizing agent for hotmetal, which method effectively reuses desulfurization slag (KR slag)resulting from a mechanical agitation type hot metal desulfurizingtreatment step, an apparatus for use in this method, a desulfurizingagent (flux) for use in this method and apparatus, and a method ofdesulfurizing hot metal using this desulfurizing agent.

[0005] 2. Description of the Related Art

[0006] Hot metal output from a blast furnace contains a highconcentration of sulfur (S), which normally affects the quality ofsteel. However, since a converter step is intended to oxidize and removeimpurities, molten steel is not expected to be desulfurized except forpart thereof which is vaporized and desulfurized. Thus, depending on thedesired quality, various hot metal pretreatments are executed betweenthe blast furnace step and the converter step or molten steel isdesulfurized after the converter step. FIG. 23 shows an example of a hotmetal pretreatment. In the illustrated example, hot metal from the blastfurnace is sequentially subjected to a desiliconizing treatment, adesulfurizing treatment, and a dephosphorizing treatment. The hot metalis then placed in the converter, where it is subjected to adecarburizing treatment.

[0007] For desulfurization, a lime-based desulfurizing agent is oftenused. A desulfurizing reaction in this case proceeds according to thereaction formula shown below.

CaO+S→CaS+O

[0008] With only CaO, such a desulfurizing reaction involves a highmelting point. Thus, typically, fluorite, alumina-based flux, or thelike is industrially used to facilitate slagging. However, theseslagging agents are generally expensive, so that an increase in thecompounding rate of such a slagging agent leads to an increase in thecosts of the desulfurizing agent. Furthermore, an increase in thecompounding rate of the slagging agent may reduce the concentration oflime in the desulfurizing agent to degrade reaction effects.

[0009] Further, slag resulting from a pyrometalurgy step in a blastfurnace or a converter is reused for blast furnace cement, concretematerial, fertilizers, or road material after having contained metalremoved. However, desulfurization slag is characterized by containing alarge amount of CaO and being likely to be weathered. Accordingly, thereis no other way but to use the desulfurizing slag for cement material byexecuting a pretreatment that requires much time and labor. Furthermore,at present, this treatment involves high cost.

[0010] Jpn. Pat. Appln. KOKAI Publication No. 4-120209 describes atechnique of utilizing converter slag as a slagging agent. Thisapplication states that the grain size of the blast furnace slag is setat 3 to 50 mm and that this range of grain size serves to produce asufficient dephosphorizing effect. However, this application is notmainly intended for desulfurization, and has no references todesulfurization.

[0011] Further, Jpn. Pat. Appln. KOKAI Publication No. 10-30115discloses a technique of cooling and crushing converter slag, separatingand recovering the iron component from the slag, and compounding limeand fluorite into the slag so that it can be used as a desulfurizingagent. However, this application has no references to the recycling ofdesulfurization slag as in the above technique.

[0012] As an example of the recycling of desulfurization slag, there isa report on a process of reusing injection desulfurization slagcontaining a large amount of unreacted lime, in a mechanical agitationtype hot metal desulfurizing treatment which allows lime to be usedefficiently (Sumitomo Metals Vol. 45-3 (1993) p. 52 to 58). However,with the treatment (hereinafter referred to as the “prior art ofdesulfurization slag recycling”) reported in this document, there is alimit to the improvement of efficiency of utilization of lime asdescribed later. Further, since this technique reuses the slag in theprocess using different addition and agitation methods, it is notapplicable if a plurality of processes are not available.

[0013] A mechanical agitation type hot metal desulfurization apparatusdesulfurizes hot metal by immersing and rotating an impeller in hotmetal, adding a desulfurizing agent (normally lime) to the hot metalfrom above, and rotating the impeller to agitate the hot metal. Aprocess commonly known as the “KR method” is a hot metal desulfurizingtreatment using this apparatus. FIG. 24 shows an example of adesulfurizing facility.

[0014] As described above, at present, desulfurization slag resultingfrom a hot metal desulfurizing treatment is not effectively recycled.Further, for desulfurization of hot metal, there are many points to beimproved.

BRIEF SUMMARY OF THE INVENTION

[0015] The inventors examined the efficiency of utilization of adesulfurizing agent used in the KR desulfurizing step. FIG. 18 shows theratio of lime effectively used to introduced lime wherein the case inwhich desulfurization slag resulting from a mechanical agitation typehot metal desulfurizing treatment method is used as a desulfurizingagent for the mechanical agitation type hot metal desulfurizingtreatment method is compared with the case in which desulfurization slagresulting from an injection method is used as a desulfurizing agent forthe mechanical agitation type hot metal desulfurizing treatment method.As indicated in FIG. 18, the inventors have found that the efficiency ofutilization of the desulfurizing agent during a single KR desulfurizingstep corresponds to about 7% of the total desulfurizing agent, with theremaining 93% unreacted. Accordingly, on the basis of this knowledge,the inventors expected that the desulfurizing agent that has been usedin the KR desulfurizing step can be reused as an inexpensive source oflime for the desulfurizing agent because it still contains about 93% oflime that contributes to desulfurization in the subsequent treatment.

[0016] The present invention is provided on the basis of this knowledge.It is an object of the present invention to provide a method ofdesulfurizing hot metal, which method effectively reuses desulfurizationslag resulting from a hot metal desulfurizing treatment in order toreduce the costs of hot metal desulfurization and the amount of slaggenerated.

[0017] It is another object of the present invention to inexpensivelycarry out a hot metal desulfurizing treatment with a reduced amount ofslag generated and to provide a desulfurizing agent for use in thistreatment.

[0018] It is yet another object of the present invention to provide amethod of transporting desulfurization slag and an apparatus used tomanufacture desulfurization slag.

[0019] The present invention is provided to accomplish these objects.

[0020] A method of manufacturing a hot metal desulfurizing agentaccording to the present invention comprises the following steps:

[0021] 1. A method of manufacturing a hot metal desulfurizing agent, themethod comprising executing a treatment for creating a new surface indesulfurization slag resulting from a mechanical agitation type hotmetal desulfurizing treatment.

[0022] 2. A method of manufacturing a hot metal desulfurizing agent foruse in a mechanical agitation type hot metal desulfurizing treatment,the method comprising executing a treatment for creating a new surfacein desulfurization slag resulting from the mechanical agitation type hotmetal desulfurizing treatment.

[0023] 3. A method comprising a step of providing desulfurization slagresulting from the mechanical agitation type hot metal desulfurizingtreatment and a step of executing the treatment for creating a newsurface in the provided desulfurization slag.

[0024] 4. A method wherein the step of executing the treatment forcreating a new surface includes crushing a desulfurization slag grainand/or separating an aggregate of a plurality of desulfurization slaggrains into desulfurization slag grains.

[0025] 5. A method wherein the step of executing the treatment forcreating a new surface includes crushing the desulfurization slag grainand/or separating the aggregate of a plurality of desulfurization slaggrains into the desulfurization slag grains by air-cooling thedesulfurization slag and/or applying mechanical energy to thedesulfurization slag.

[0026] 6. A method wherein the desulfurization slag is cooled using oneor two methods selected from a group consisting of air cooling and watercooling.

[0027] 7. A method wherein the air cooling is carried out using one ortwo methods selected from a group consisting of natural cooling andforced air cooling.

[0028] 8. A method wherein the step of executing the treatment forcreating a new surface by water cooling comprises a step of watering thedesulfurization slag, and this watering step controls the amount ofwater provided so as to maintain the temperature of the desulfurizationslag at 100° C. or higher at the end of the watering, so that only bymeans of cooling based on the watering, the aggregate of desulfurizationslag can be separated into desulfurization slag grains and/or thedesulfurization slag grains can be crushed.

[0029] 9. A method wherein the step of executing the treatment forcreating a new surface comprises a step of water-cooling thedesulfurization slag and a step of drying the recovered desulfurizingagent resulting from the water cooling.

[0030] 10. A method wherein the step of executing the treatment forcreating a new surface comprises a step of cooling the desulfurizationslag and a step of adjusting the grain sizes of the desulfurization slagand the recovered desulfurizing agent.

[0031] 11. A method wherein the step of adjusting the grain size of therecovered desulfurizing agent using a sieve is executed at a temperatureof 600° C. or higher.

[0032] 12. A method wherein the step of executing the treatment forcreating a new surface comprises a step of executing at least one or twotreatments selected from a group consisting of a treatment formagnetically separating and removing metal from the desulfurization slagor the recovered desulfurizing agent, a treatment for removing largemasses from the desulfurization slag or the recovered desulfurizingagent to set the grain size at 100 mm or smaller, and a treatment forsetting the temperature of the desulfurization slag or the recovereddesulfurizing agent at 200° C. or lower.

[0033] 13. A method wherein the step of executing the treatment forcreating a new surface comprises a step of setting the grain size of thedesulfurization slag at 100 mm or smaller and the temperature thereof at200° C. or lower.

[0034] 14. A method of transporting a recovered desulfurizing agent, themethod comprising a step of loading a recovered desulfurizing agentresulting from a mechanical agitation type hot metal desulfurizingtreatment onto a transport vehicle using a pair of movable basketsections that can be opened and closed and a step of using the transportvehicle to transport the recovered desulfurizing agent to adesulfurizing treatment facility.

[0035] 15. A method comprising a step of screening the cooled andcrushed recovered desulfurizing agent to remove large masses before orsimultaneously with the step of using the transport vehicle to transportthe recovered desulfurizing agent.

[0036] 16. A method of transporting a recovered desulfurizing agent, themethod comprising a step of loading a recovered desulfurizing agentresulting from a mechanical agitation type hot metal desulfurizingtreatment onto a transport vehicle having a capability of sucking therecovered desulfurizing agent and a step of using the transport vehicleto transport the recovered desulfurizing agent to a desulfurizingtreatment facility.

[0037] 17. A method wherein the step of loading the recovereddesulfurizing agent is executed while adjusting a height from which therecovered desulfurizing agent is dropped onto the transport vehicle, to1.5 m or less.

[0038] 18. An apparatus which screens a recovered desulfurizing agent,the apparatus comprising an apparatus main body having a sieve mesh thatscreens a crushed recovered desulfurizing agent and an air sucking hoseattached to the apparatus main body to facilitate suction of therecovered desulfurizing agent into the apparatus main body.

[0039] 19. An apparatus which screens a recovered desulfurizing agent,the apparatus comprising a member having a diagonally arranged sievemesh, a diagonal plate diagonally arranged below the member and on whicha minus sieve slides down, and a sliding way on which the minus sievefrom the diagonal plate falls and then slides down, and wherein theapparatus is arranged so that the spacing between the member having thesieve mesh and the diagonal plate and the vertical height of drop of alinkage between the diagonal plate and the sliding way are each 500 mmor less and the height of drop from the sliding way to a ground surfaceis 500 mm or less.

[0040] 20. A hot metal desulfurizing agent comprising a recovereddesulfurizing agent resulting from a mechanical agitation type hot metaldesulfurizing treatment.

[0041] 21. A hot metal desulfurizing agent comprising the recovereddesulfurizing agent resulting from the mechanical agitation type hotmetal desulfurizing treatment, the hot metal desulfurizing agent beingused for the mechanical agitation type hot metal desulfurizingtreatment.

[0042] 22. A hot metal desulfurizing agent comprising the recovereddesulfurizing agent resulting from the mechanical agitation type hotmetal desulfurizing treatment and having a new surface created therein.

[0043] 23. A hot metal desulfurizing agent comprising the recovereddesulfurizing agent which results from the mechanical agitation type hotmetal desulfurizing treatment and in which part or all of an aggregateof recovered desulfurizing agent grains is separated into pieces.

[0044] 24. A hot metal desulfurizing agent having a maximum grain sizeof 100 mm or smaller.

[0045] 25. A hot metal desulfurizing agent comprising the recovereddesulfurizing agent resulting from the mechanical agitation type hotmetal desulfurizing treatment, the hot metal desulfurizing agent furthercontaining one or two sources selected from a group consisting of a limesource and a carbon source.

[0046] 26. A hot metal desulfurizing agent wherein the one or twosources selected from the group consisting of the lime source and thecarbon source are mixed with the recovered desulfurizing agent resultingfrom the mechanical agitation type hot metal desulfurizing treatment,and this mixture is added to hot metal.

[0047] 27. A hot metal desulfurizing agent wherein the one or twosources selected from the group consisting of the lime source and thecarbon source are separated from the recovered desulfurizing agentresulting from the mechanical agitation type hot metal desulfurizingtreatment, and the one or more sources and the recovered desulfurizingagent are separately added to hot metal.

[0048] 28. A hot metal desulfurizing agent wherein the lime source is atleast one or two selected from a group consisting of lime, calciumcarbonate, and calcium hydroxide.

[0049] 29. A hot metal desulfurizing agent wherein the total amount ofcalcium carbonate and calcium hydroxide, which are included in the groupof lime sources, corresponds to 40 wt % or less of the whole hot metaldesulfurizing agent.

[0050] 30. A hot metal desulfurizing agent wherein the amount of carbonsource corresponds to 30 wt % or less of the whole hot metaldesulfurizing agent.

[0051] 31. A hot metal desulfurizing agent wherein the carbon source ispowders of grain size 1 mm or smaller.

[0052] 32. A hot metal desulfurizing agent wherein the carbon source isat least one or two selected from a group consisting of coal, coke, andpitch.

[0053] 33. A method of manufacturing low-sulfur hot metal, the methodcomprising desulfurizing hot metal by adding, to the hot metal, a hotmetal desulfurizing agent comprising a recovered desulfurizing agentresulting from a mechanical agitation type hot metal desulfurizingtreatment.

[0054] 34. A method of manufacturing low-sulfur hot metal, the methodcomprising desulfurizing hot metal using a mechanical agitation type hotmetal desulfurizing treatment, by adding, to the hot metal, a hot metaldesulfurizing agent comprising a recovered desulfurizing agent resultingfrom the mechanical agitation type hot metal desulfurizing treatment.

[0055] 35. A method of manufacturing low-sulfur hot metal, the methodcomprising desulfurizing the hot metal by adding, to the hot metal, thedesulfurizing agent comprising the recovered desulfurizing agentresulting from the mechanical agitation type hot metal desulfurizingtreatment and having a new surface created therein.

[0056] 36. A method of manufacturing low-sulfur hot metal, the methodcomprising desulfurizing the hot metal by adding, to the hot metal, thedesulfurizing agent comprising the recovered desulfurizing agent whichresults from the mechanical agitation type hot metal desulfurizingtreatment and in which part or all of an aggregate of the recovereddesulfurizing agent is separated into pieces.

[0057] 37. A method comprising desulfurizing the hot metal by adding, tohot metal, the recovered desulfurizing agent resulting from themechanical agitation type hot metal desulfurizing treatment and one ormore sources selected from a group consisting of a lime source and acarbon source.

[0058] 38. A method comprising desulfurizing the hot metal by mixing therecovered desulfurizing agent resulting from the mechanical agitationtype hot metal desulfurizing treatment with one or more sources selectedfrom a group consisting of a lime source and a carbon source, and addingthis mixture to hot metal.

[0059] 39. A method comprising desulfurizing the hot metal by separatelyadding, to the hot metal, the recovered desulfurizing agent resultingfrom the mechanical agitation type hot metal desulfurizing treatment andone or more sources selected from a group consisting of a lime sourceand a carbon source.

[0060] 40. A method wherein when the lime source is added, the mixingratio is adjusted so as to obtain a predetermined amount of pure CaOcomponent.

[0061] 41. A method comprising a step of calculating the bulk density ofthe recovered desulfurizing agent resulting from the mechanicalagitation type hot metal desulfurizing treatment, a step of calculatingthe amount of pure CaO component in the recovered desulfurizing agentfrom the calculated bulk density, and a step of adjusting the additionratio of the crushed recovered desulfurizing agent to the lime source onthe basis of the calculated amount of pure CaO component in therecovered desulfurizing agent.

[0062] 42. A method comprising a step of adjusting the grain size of thecarbon source to 1 mm or smaller when the carbon source is added.

[0063] Definition

[0064] “Desulfurizing agent” refers to common fluxes used fordesulfurization. It includes slag recovered after a treatment forcreating a new surface.

[0065] “Recovered desulfurizing agent” refers specifically to adesulfurizing agent comprising slag recovered after the treatment forcreating a new surface, wherein the slag may contain metal.

[0066] “Desulfurization slag” refers to desulfurization slag containingall of a CaO component, other slag components, and a metal component andwhich has not been subjected to the treatment for creating a newsurface.

[0067] “Lime” refers to common CaO components.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0068]FIG. 1 is a graph illustrating the relationship between the amountof lime added per unit amount of hot metal and desulfurization rate, forrecovered desulfurizing agents resulting from the first and secondrecycling operations, respectively, according to the present inventionas well as a conventional lime desulfurizing agent for comparison.

[0069]FIG. 2 is a graph illustrating the relationship between the numberof times of recycling processes executed on desulfurization slag and theamount of desulfurizing agent used per unit amount of hot metal whereinthe slag is recycled a number of times according to the presentinvention.

[0070]FIG. 3 is a graph illustrating the relationship between the amountof desulfurizing agent added per unit amount of hot metal and thedesulfurization rate, for the desulfurizing agent according to thepresent invention and the conventional desulfurizing agent forcomparison.

[0071]FIG. 4 is a graph illustrating the relationship between the amountof lime component added per unit amount of hot metal and thedesulfurization rate, for the desulfurizing agent according to thepresent invention and the conventional desulfurizing agent forcomparison.

[0072]FIG. 5 is a graph illustrating the relationship between the amountof lime added per unit amount of hot metal and the desulfurization rate,for the recovered desulfurizing agent treated according to the presentmethod and the conventional desulfurizing agent for comparison.

[0073]FIG. 6 is a graph illustrating the relationship between thetemperature of slag measured at the end of watering and the timerequired to cool the slag down to room temperature, wherein a treatmentis executed using a desulfurization slag treatment method according tothe present invention.

[0074]FIG. 7 is a graph illustrating the relationship between thetemperature of the slag measured at the end of watering and the amountof Ca(OH)₂ generated in the recovered desulfurizing agent, wherein atreatment is executed using the desulfurization slag treatment methodaccording to the present invention.

[0075]FIG. 8 is a graph illustrating the relationship between the timerequired to cool slag with different thicknesses and the temperature ofthe slag, wherein a treatment is executed using the desulfurization slagtreatment method according to the present invention.

[0076]FIG. 9 is a graph showing the relationship between the amount oflime added per unit amount of hot metal and desulfurization rate, forthe recovered desulfurizing agent treated according to the presentinvention and the conventional desulfurizing agent for comparison.

[0077]FIG. 10 is a graph showing the relationship between the amount oflime added per unit amount of hot metal and desulfurization rate, forthe recovered desulfurizing agent recycled according to the presentinvention and the conventional desulfurizing agent for comparison.

[0078]FIG. 11 is a view showing an example of a screening jig accordingto the present invention.

[0079]FIG. 12 is a view showing a screening facility according to thepresent invention.

[0080]FIG. 13A is a view useful in describing an essential part of arecovered desulfurizing agent loading apparatus, and FIG. 13B is aschematic view showing the entire recovered desulfurizing agent loadingapparatus.

[0081]FIG. 14 is a graph showing the amount of pure CaO component in thedesulfurizing agent and the amount of material desulfurized (the amountof sulfur before treatment (S)—the amount of sulfur after treatment(S)).

[0082]FIG. 15 is a graph showing the bulk density and CaO mass % ofdesulfurization slag.

[0083]FIG. 16 is a graph showing the relationship between the amount oflime added per unit amount of hot metal and the desulfurization rate,for desulfurizing agents obtained by mixing the recovered desulfurizingagent recovered according to the present invention with lime and aconventional desulfurizing agent as a comparative example.

[0084]FIG. 17A is a view showing a slag treatment pattern performed by aplant test.

[0085]FIG. 17B is a view showing an example of the slag treatmentpattern in FIG. 17A together with a conventional slag treatment pattern.

[0086]FIG. 18 is a graph showing the ratio of lime effectively used fordesulfurization to introduced lime, by comparing the case in whichdesulfurizing agent resulting from a mechanical agitation type hot metaldesulfurizing treatment method is used as a desulfurizing agent for themechanical agitation type hot metal desulfurizing treatment with thecase in which desulfurization slag resulting from an injection method isused as a desulfurizing agent for the mechanical agitation type hotmetal desulfurizing treatment method.

[0087]FIG. 19 is a graph showing the relationship between the amount oflime component added per unit amount of hot metal and thedesulfurization rate at each desulfurization level.

[0088]FIG. 20 is a graph showing the relationship between the amount oflime component added per unit amount of hot metal and thedesulfurization rate at each desulfurization level.

[0089]FIG. 21 is a photograph showing how an aggregate ofdesulfurization slag resulting from the mechanical agitation type hotmetal desulfurizing treatment was observed using an SEM, and a viewshowing the results of line analysis of an S element in the aggregate.

[0090]FIG. 22 is a view schematically showing differences betweenagitating type hot metal desulfurization slag and hot metaldesulfurization slag obtained by the injection method.

[0091]FIG. 23 is a view showing an example of a hot metal pretreatment.

[0092]FIG. 24 is a view showing an example of a desulfurizing facility,shown in FIG. 23.

DETAILED DESCRIPTION OF THE INVENTION

[0093] Recycling of Desulfurization Slag

[0094] According to a hot metal desulfurizing method of the presentinvention, desulfurization slag resulting from mechanical agitation typehot metal desulfurizing treatment (hereinafter referred to as the “KRmethod”) is reused as a desulfurizing agent for another hot metaldesulfurizing treatment (for example, desulfurization slag resultingfrom the KR method is reused as a desulfurizing agent for the KR method,or desulfurization slag resulting from the KR method is reused as adesulfurizing agent for an injection method). The hot metaldesulfurizing treatment for which desulfurization slag is reused is notlimited but refers to a commonly executed one. According to the presentinvention, desulfurization slag can also be reused for the same processthat results in the desulfurization slag. Accordingly, the method of thepresent invention is effective even in steel manufacturing facilitiesprovided with only hot metal desulfurizing treatment processes that usethe same addition method and agitation method. The present inventionallows desulfurization slag to be particularly efficiently andeffectively recycled if the recycling operation is applied to the KRmethod.

[0095] That is, with the injection method, fine lime powders are addedto a bath at a deep position thereof, and thus react while floating inthe bath. Consequently, the fine powders are expected to react for onlya short time, and a desulfurization product is formed in front layers ofthe fine powders to only a small thickness. After the fine powders havecome floating to the bath surface, they are not expected be agitatedagain so as to be caught in waves of hot metal. Thus, a reaction surfacearea does not increase, and there are substantially no possibilities ofdesulfurizing reaction. Further, at this stage, aggregation starts, sothat desulfurization products on the surfaces of the individual finepowders are aggregated.

[0096] On the other hand, with the KR method, lime powders are added tothe bath surface. Accordingly, the lime powders are caught in waves ofhot metal in so as to go down from the surface to interior of the bath.Further, at the beginning of the addition, desulfurizing agent powdersstart to be aggregated near the surface. As a result, the lime powersbecome “undissolved masses” with substantially unreacted lime containedtherein. Even when aggregation starts, surface areas of the powderswhich come into contact with metal react, and desulfurization productsare formed thereon. This reaction continues over the treatment time, andlong-time reaction is thus possible. With the above reaction mechanism,after desulfurization, the surfaces of the aggregated coarse grains arecovered with desulfurization products to a fixed thickness. There areonly a small amount of desulfurization products inside the grains. Thatis, unreacted lime, which is similar to fresh lime, is present insidethe grains. The inventors used an SEM to observe an aggregate ofdesulfurization slag resulting from the mechanical agitation type hotmetal desulfurizing treatment, took photographs thereof, andline-analyzed an S element therein. As a result, the inventors confirmedthat the above knowledge is correct, as shown in FIG. 21.

[0097] Thus, desulfurization slag resulting from the KR method containscoarse grains and thus requires only a simple pretreatment forrecycling, thereby reducing recycling treatment costs. Further, in mostcases, unreacted lime grains are aggregated and can thus be recycledwithout using any additional special methods of crushing material oradjusting grain size.

[0098] On the other hand, desulfurization slag resulting from theinjection method,, the individual fine grains are enclosed bydesulfurization products. Accordingly, a special crushing operation orthe like is required to make the fine grains much finer. This increasesthe number of steps and the treatment time.

[0099] As described above, the KR method accomplishes a high reactionefficiency during recycling. The surface of an unreacted lime componentcan be used directly for reaction for recycling, and is expected toachieve a reaction efficiency equivalent to that attained using freshlime. If KR slag is recycled, it can be desulfurized with the amount oflime component added per unit amount of hot metal, unreacted. Incontrast, with injection slag, a complicated pretreatment is required,or the use of a simple pretreatment requires at least double the amountof lime component.

[0100] Further, the use of KR slag reduces the treatment time (down tothe same amount as that required for a fresh agent) and effectivelyreduces the amount of slag generated. The KR slag serves to reduce theamount of slag generated, for example, by 50% after one recyclingprocess and to allow recycling to be carried out a number of times.

[0101] On the other hand, with the injection method, desulfurizationproducts on the surfaces of individual fine grains are aggregated. Thus,for recycling, the individual grains (primary grains) must be crushed.After recycling, the grains are very fine (their grain size is half orsmaller of that of the primary grain size). Thus, during recycling basedon the KR method, a large number of grains are likely to scatter.Consequently, additional lime is required to make up for the loss of thescattered lime component. For example, if only a conventionaldesulfurizing agent resulting from the first recycling operation isused, about 7 kg/t of desulfurizing agent (about 90% of lime component:6.3 kg/T of substantial lime component) is required. In contrast, if arecovered desulfurizing agent is used, 10 kg/t of desulfurizing agent(about 66% of lime component: 6.6 kg/T of substantial lime component)+3kg/t of conventional desulfurizing agent resulting from the firstrecycling operation (about 90% of lime component: 2.7 kg/T ofsubstantial lime component) is required. That is, 9.3 kg/T of limecomponent is present in the recovered desulfurizing agent, but becauseof insufficient formation of effective new surfaces or a loss resultingfrom scattering, about 3 kg/T of desulfurizing agent may not effectivelycontribute to desulfurization. That is, an effective lime componenttakes up only about 55% of the recovered desulfurizing agent, so that asupplementary desulfurizing agent must be added, as described in thepreviously described document (Sumitomo Metals Vol. 45-3 (1993) p.52 to58).

[0102] With the KR method, as described above, the surface of coarsegrains are covered with desulfurization products to some degree, withonly a small amount of desulfurization products present inside thegrains. That is, unreacted lime similar to fresh lime is present. Thus,desulfurization is possible if weathering products are positivelygenerated for recycling and even if the grains after recycling are aslarge as or larger than the primary grains. Furthermore, duringrecycling based on the KR method, the amount of grains scattered isequal to or smaller than that observed when lime is used. For example,if a recovered desulfurizing agent is used, the amount of recovereddesulfurizing agent used is 14 kg/t (about 50% of lime), and the amountof effective lime component in the added recovered desulfurizing agentis similar to that observed if fresh lime is added.

[0103]FIG. 18 shows a summary of the above relationships. Ifdesulfurization slag resulting from the KR method, an example of thepresent invention, is reused as a desulfurizing agent for the KR method,about 7% of the lime component added for one treatment is used fordesulfurization. Effective utilization rate increases consistently withthe number of times that the desulfurizing agent is used. In contrast,if desulfurization slag resulting from the injection method, aconventional technique for desulfurization slag recycling, is reused asa desulfurizing agent for the KR method, the total utilizationefficiency only slightly exceeds the utilization efficiency accomplishedby one KR process.

[0104] Further, to help make the reader understand the abovedescription, the specification is accompanied by FIG. 22 schematicallyshowing differences between hot metal desulfurization slag resultingfrom the mechanical agitation type method and hot metal desulfurizationslag resulting from the injection method. FIG. 22 shows how an aggregateof desulfurization slag grains resulting from the mechanical agitationtype hot metal desulfurizing treatment is separated into pieces tocreate new surfaces and how each grain of aggregate of desulfurizationslag grains resulting from the injection method is crushed into pieces.In the figure, the desulfurization slag grains resulting from theinjection method have substantially the same grain size as thedesulfurization slag grains resulting from the mechanical agitation typehot metal desulfurizing treatment. However, the desulfurization slaggrains resulting from the injection method are actually finer.

[0105] Specific Treatment for Desulfurization Slag

[0106]FIG. 17A shows a slag treatment pattern performed by a plant test,and FIG. 17B shows an example of the slag treatment pattern in FIG. 17Atogether with a conventional slag treatment pattern.

[0107] According to the present invention, an arbitrary method is usedto create new surfaces in desulfurization slag resulting from adesulfurizing step, and the slag obtained is reused as a desulfurizingagent. In this case, an unreacted lime component must be exposed as adesulfurization reacting surface so that the slag can be used as adesulfurizing agent for the next process. The method used to achievethis is not limited. If a natural cooling process or a watering andcooling process is executed to create new surfaces, CaCO₃ and Ca(OH)₂are generated. The residual CaCO₃ and Ca(OH)₂ do not hinderdesulfurization reaction, but an appropriate amount of these componentsare expected to serve to improve this reaction. Further, a metalcomponent of a larger diameter can be removed by magnetic separation orscreening, so that a recovered desulfurizing agent is mainly composed ofa lime component. Furthermore, the grain size of this recovereddesulfurizing agent is restricted by a supply apparatus in adesulfurizing facility where the agent is used. Accordingly, no problemsoccur provided that an appropriate grain size is used. On the otherhand, residual metal of a smaller diameter may remain in the recovereddesulfurizing agent. However, this metal can be reused as an iron sourcefor the subsequent hot metal pretreatment step, thereby significantlycontributing to increasing the yield of iron. Various specific exampleswill be illustrated below.

[0108] (i) Crushing Based on Watering Treatment

[0109] In this example, desulfurization slag resulting from adesulfurizing step is simultaneously cooled and crushed using a wateringtreatment, and is then dried and reused as a desulfurizing agent.Specifically, a watering facility is used to excessively water hot slagresulting from the desulfurizing process until the slag is fullyimpregnated with water. Subsequently, a drying apparatus is used tocompletely dry this water-containing slag to obtain a desulfurizingagent having a reduced grain size of 100 mm or smaller. However, asmaller grain size is more preferable, and the maximum grain size ispreferably substantially 30 mm or smaller and more preferably 5 mm orsmaller. A mechanical crushing operation may be performed before orafter the watering and drying steps. Further, in an actual process,mechanical vibration causes at least part of desulfurization slag to becrushed during transportation. Specifically, an apparatus used fordrying may be a dryer, or a rotary kiln or the like may be used toperform a large-scale drying operation. The size of the apparatus andthe like can be set depending on a required throughput or the like. Anyapparatus and method may be used provided that water impregnated intothe cooled slag can be sufficiently removed. The thus recycleddesulfurization slag is used as a desulfurizing agent.

[0110] (ii) Crushing Based on Watering and Agitating Treatment

[0111] In this example, desulfurization slag resulting from thedesulfurizing step is simultaneously cooled and crushed using anappropriate watering and agitating treatment, and is then reused as adesulfurizing agent. That is, a watering facility is used to uniformlywater hot slag resulting from the desulfurizing treatment, while usingheavy equipment such as a shovel to agitate the slag. Specifically, awatering operation is performed until the hot slag is cooled down to atemperature of about 100° C. The slag is then left cooled to obtain adesulfurizing agent of a reduced grain size of about 100 mm or smaller.However, a smaller grain size is more preferable, and the maximum grainsize is preferably substantially 30 mm or smaller and more preferably 5mm or smaller. A mechanical crushing operation may be performed beforeor after the watering step. The cooling target temperature is notlimited to a particular value, but may be set depending on the requiredthroughput or the like. An appropriate amount of water supplied andagitation reduces the time required to cool the slag. However, if thewatering operation is continued even after the temperature decreasesbelow 100° C., then a drying treatment is required. Thus, the wateringoperation is desirably stopped before the temperature reaches 100° C.Further, the agitation is carried out in order to increase cooling speedand make the watering uniform, so that its frequency may be varieddepending on the required treatment time or throughput. The agitationmay be omitted.

[0112] (iii) Crushing Based on Natural Cooling

[0113] In this example, desulfurization slag resulting from thedesulfurizing step is simultaneously cooled and crushed by executingnatural cooling. The resultant slag is reused as a desulfurizing agent.That is, hot slag resulting from the desulfurizing step is left as it isso as to maximize the area of that part of the slag which contacts withair. The slag is then agitated using heavy equipment such as a shovel.Specifically, a recovered desulfurizing agent of temperature 200° C. orlower and a sufficiently reduced diameter can be obtained in three daysby spreading the hot slag so that its thickness is 0.5 m or smaller andagitating it about one to three times a day. The thickness of the slagduring cooling is not limited to this value, but the target thicknesscan be set depending on the required treatment time or throughput, thearea of a place that is available for the recycling treatment, or thelike. Further, the agitation is carried out in order to increase thecooling speed, so that its frequency may also be varied depending on therequired treatment time or throughput. The agitation may be omitted ifmuch treatment time and throughput are available. Furthermore, arecovered desulfurizing agent is similarly obtained if the slag is leftas it is without reducing its thickness.

[0114] The cooled and crushed desulfurization slag grains have a maximumgrain size of 100 mm or smaller, preferably 30 mm or smaller, and morepreferably 5 mm. Thus, these grains have a surface area sufficient to beinvolved in a desulfurizing reaction and can be advantageously handledbecause they do not scatter upon addition owing to their larger size. Inaddition, a mechanical crushing operation may also be used.

[0115] (iv) Screening Hot Slag

[0116] In this example, while hot at 900 to 1,200° C., desulfurizationslag resulting from the desulfurizing step is screened using a sieve(30×30 mm to 100×100 mm) and is thus separated into metal of a largerdiameter containing a slag component and desulfurization slag of asmaller diameter. The criterion for the screening is restricted by thesupply apparatus operated when the slag is used as a recovereddesulfurizing agent. Typically, the above ranges are appropriate.

[0117] After the screening, the desulfurization slag of a reduceddiameter is naturally cooled and reused as it is. Even after thescreening, about 20 to 30% of Fe component (T.Fe from the slag andmetallic Fe) remains. However, this Fe component is introduced into hotmetal when it is used for the next desulfurizing step, therebyincreasing the yield of iron.

[0118] When the metal of a larger diameter containing the above slagcomponents is cooled, an unreacted lime component in the slag componentcauses the following reaction:

CaO+H₂O=Ca (OH)₂

CaO+CO₂=CaCO₃

[0119] i.e. so-called “powdering” reaction, thereby disintegrating theslag component in the desulfurization slag into a metal component and aslag component. After this step, the above screening operation may beperformed again, so that the desulfurization slag can be efficientlyseparated into large masses of a metal component (containing a smallamount of slag component) and a recovered desulfurizing agent(containing a metal component of a smaller diameter). As a result, about90% of the unreacted lime component in the desulfurization slag isrecovered as a recovered desulfurizing agent.

[0120] In this case, any sieve can be used without creating any problemsprovided that it can be operated over a temperature range from 900 to1,200° C.; a sieve made of iron is sufficient. The shape, specification,and the like of this sieve are not limited provided that it can beoperated in a desulfurization slag treatment field. Further, the mesh ofthe sieve is restricted by the supply apparatus in the desulfurizingfacility where the slag is used as a recovered desulfurizing agent. Anappropriate mesh can be used without creating any problems.

[0121] As described above, desulfurization slag can be recovered as aninexpensive lime source by exposing an unreacted lime component andseparating large masses of metal from the desulfurization slag withoutmagnetic separation. Furthermore, a recovered desulfurizing agent mainlycomposed of the desulfurization slag recovered after screening still hasa sufficient desulfurizing capability. Therefore, lime can be moreeffectively utilized by reusing the desulfurization slag a number oftimes.

[0122] (v) Suppression of Dust

[0123] Dried desulfurization slag is very likely to produce dust.However, the inventors note that since desulfurization slag is hotimmediately after being produced, most of the slag pieces are massive.That is, a large amount of dust generated can be prevented by properlyhandling the desulfurization slag, e.g. screening it while it is hotafter being generated. A higher temperature is more advantageous tosuppression of dust. Slag was actually dropped from a drop height of 3 mwith its temperature varied, and the amount of dust generated wasmeasured. Then, it was found that the amount of dust increases rapidlyat a temperature of 600° C. Further, in connection with an incidentaleffect of screening at high temperature, by exposing desulfurizationslag to the atmosphere at high temperature, the desulfurization slag canbe cooled rapidly and thus powdered. This reduces the subsequent coolingload.

[0124] Further, in connection with a facility used for screening, theamount of dust produced can be reduced using a facility that serves toreduce the height from which desulfurization slag is dropped vertically.FIG. 12 shows the configuration of this facility.

[0125] This facility is composed of a diagonally arranged net (12), adiagonal plate on which an undersize slides down, and a sliding way(16). This facility is characterized in that the spacing (13) betweenthe net (12) and the diagonal plate (14) is set at 500 mm or smaller inorder to reduce the drop height from which desulfurization slag isdropped. Further, the spacing (15) between the diagonal plate (14) andthe sliding way (16) is set at 500 mm or smaller. Furthermore, thefacility is configured so that the drop height from the sliding way (16)to the ground (18) is set at 1,500 mm or less. This configuration servesto drastically reduce the amount of dust produced. Thus, the amount ofdust produced during the treatment can also be reduced by using theabove configuration to adjust the grain size (e.g. 70 mm or smaller) ofdried desulfurization slag using the simple facility and low operationcosts and further screening the slag while it is at a temperature of600° C. or higher.

[0126] Further, if the desulfurization slag having its grain sizeadjusted as described above is cooled and then loaded onto a dump truckfor recycling, then naturally, it is powdered faster and is very likelyto produce dust. Thus, desulfurization slag is loaded using a facilitysuch as the one shown in FIG. 13. A movable basket section (21) isopened in opposite directions and picks up a recovered desulfurizingagent (22). The movable basket section (21) is then closed and carriesthe picked-up recovered desulfurizing agent to immediately above acarrier (23) of a dump truck, where the movable section (21) is opened.In this case, the drop height (24) is 1.5 m or less so that thedesulfurization slag can be loaded onto the dump truck whilesignificantly reducing the amount of dust produced.

[0127] (vi) Suppression of Other Dust

[0128] If a recovered dry desulfurizing agent is loaded onto a transportvehicle such as a dump truck for recycling as described above, thennaturally, it is powdered faster and is very likely to produce dust.However, the amount of dust produced can be minimized by using a vehiclewith a sucking capability instead of the dump truck to transport therecovered desulfurizing agent. Further, the recovered desulfurizingagent can concurrently be sieved by installing a mesh at a suction portduring suction, thereby enabling efficient handling and transportationwith a reduced amount of dust produced.

[0129] Specifically, a suction hose is connected to the vehicle with thesucking capability to suck and load a recovered desulfurizing agentobtained by cooling and crushing desulfurization slag resulting from thehot metal desulfurizing step. At this time, by installing a jig having amesh for screening at the suction port, the desulfurizing agent canconcurrently be sieved so that the required grain size can be obtained.In this case, the jig attached to the suction port during suction may beselected on the basis of the required level of the sieve, i.e. thedesired grain size of the recovered desulfurizing agent.

[0130] Furthermore, if no screening operation is required duringsuction, the mesh need not be installed at the suction port, and thedesulfurizing agent may be sucked using only the hose.

[0131] Moreover, if only large masses of a several tens of cm level mustbe removed, a simple jig may be used for suction, jig simply havingpartitions arranged at the tip thereof and formed of metal bars as shownin FIG. 11. An example of such a jig is shown in FIG. 11. This jig issimply composed of a cylinder (1) having the same diameter as that ofthe hose and metal bars (2) attached to the tip of the cylinder so as toform a cone. The size of the mesh of the sieve and the shape of the jigare not limited and are determined depending on the required grain size,pretreatment step, or the like. Furthermore, to facilitate suction, anozzle (3) is attached to the jig to suck air. This sucking jig is usedto suck the recovered desulfurizing agent, thereby simultaneouslyachieving the loading and screening operations.

[0132] The cooled and crushed recovered desulfurizing agent grainsobtained as described above can solely constitute a desulfurizing agentaccording to the present invention or by being combined with anothercomponent as required. That is, the desulfurizing agent according to thepresent invention may be composed of only the recovered desulfurizingagent grains obtained through the above described steps or may be formedby combining these grains with another component such as lime orfluorite, as required. In the latter case, the amount of anothercomponent mixed can be properly determined on the basis of the amount oflime component contained in the recovered desulfurizing agent or therequired desulfurization rate. A preferred determining method will bedescribed later.

[0133] Manufacture of Optimum Desulfurizing Agent

[0134] Now, description will be given of a method of manufacturing theoptimum desulfurizing agent using a recovered desulfurizing agentobtained from desulfurization slag. Even with the same recovereddesulfurizing agent, the desulfurization rate varies significantlydepending on the amount of material added per unit amount of hot metal.However, it has been found that the amount of pure CaO component in therecovered desulfurizing agent has a clear correlationship with theamount of material desulfurized (the amount of S before treatment—theamount of S after treatment).

[0135] Thus, even if the components of the recovered desulfurizing agentchange, a stable desulfurization rate is obtained by also introducing apure CaO component into the desulfurizing agent. Then, the ratio of CaOto the recovered desulfurizing agent must be determined. To achievethis, the means described below can be used. Desulfurization slag isroughly divided into a metal component and a slag component. Theinventors note that the ratio of the metal component to the slagcomponent varies significantly, whereas the ratio of CaO to the slagcomponent does not vary significantly. That is, the amount of CaOcontained can be estimated by determining the ratio of the metalcomponent to the slag component in the recovered desulfurizing agent.Furthermore, the metal has a relatively large specific gravity of about7, whereas the slag has a specific gravity of only 2 to 3.

[0136] This indicates that bulk density increases in proportion to theamount of metal contained. On the basis of this nature, it is easy tocut out a specified volume of recovered desulfurizing agent immediatelybefore use and then weigh it. This weight can be easily reflected in theamount of material introduced. Thus, according to the present invention,the amount of CaO contained in the recovered desulfurizing agent can beestimated promptly and precisely to determine the amount of recovereddesulfurizing agent required for desulfurization.

[0137] It has been found that when a desulfurizing agent obtained fromdesulfurization slag is used, even if the components of the recovereddesulfurizing agent change, a stable desulfurization rate is obtained byalso introducing a pure CaO component into the desulfurizing agent.Accordingly, if the recovered desulfurizing agent contains only a smallamount of pure CaO component, the amount of this component added mayincrease. Consequently, the hot metal temperature becomes inappropriate,the amount of slag discharged increases, and other adverse effects areproduced.

[0138] Thus, the amount of desulfurizing agent can be reduced by using adesulfurizing agent containing a mixture of a recovered desulfurizingagent with at least one or two of lime (CaO), quick lime (calciumcarbonate CaCO₃), and slaked lime (calcium hydroxide Ca(OH)₂). Ifcalcium carbonate (CaCO₃) and calcium hydroxide (Ca(OH)₂) are added,fine fluxes are generated in molten metal owing to the decompositionreaction shown below. Thus, the reaction surface area increases toimprove the desulfurization reaction rate. On the other hand, thisdecomposition reaction is of an endothermal type and causes oxides to begenerated, which hinder desulfurization reaction, which is of a reducingtype. Accordingly, addition of a large amount of calcium carbonate andcalcium hydroxide may reduce the hot metal temperature or hinder thedesulfurization reaction. Thus, the amount of these components addedmust be 40 wt % or less.

[0139] Further, the calcium carbonate component may be added byadjusting, when lime is calcined, the level of the calcination so as toleave the calcium carbonate component in the lime. Also in this case,the calcium carbonate component is mixed with the desulfurizing agent sothat the total amount of calcium carbonate in the desulfurizing agent is40 wt % or less. If two or more components are mixed with thedesulfurizing agent, the total amount of calcium carbonate component,quick lime, and slaked lime is 40 wt % or less of the total amount ofdesulfurizing agent. The amount of lime added is not limited and can befreely adjusted depending on the temperature of the hot metal to betreated.

[0140] Furthermore, if these components are added, the same effects areproduced whether predetermined amounts of these components are mixedtogether before addition or are separately added to the hot metal. Inparticular, if the amount of material desulfurized is to be increased ina short time, addition of CaO or the like is more effective if forexample, the temperature of the hot metal to be treated is low.

[0141] Further, the desulfurizing capability of the desulfurizing agentcan further be improved by adding a C source to this recovereddesulfurizing agent.

[0142] If a C source is added, it acts as a reducing agent to effect thereaction shown below. This facilitates desulfurization to increasedesulfurization reaction efficiency.

CaO+S+C→CaS+CO   (1)

[0143] Further, part of the C source dissolves in the hot metal toincrease the amount of heat source that increase the temperature inconnection with decarbonization in a converter. Addition of an excessiveamount of C source may cause an extra amount of C source to remain, theextra amount exceeding the amount of C source that is required for theabove reduction reaction or that needs to dissolve in the hot metal.Consequently, the amount of slag may increase to degrade the quality ofoperations or the environment. Therefore, the amount of C source addedto the desulfurizing agent is desirably 30 wt % or less. Further, the Csource used may contain a sulfur component, so that the concentration ofS in the hot metal may increase depending on the amount of C sourceadded. In particular, the amount of C source dissolving in the hot metalvaries with the temperature of the hot metal. Accordingly, the amount ofC source added can be adjusted on the basis of the type of the C sourceused, the temperature of the hot metal to be treated, the concentrationof S in the hot metal, the required amount of material desulfurized, thetreatment time, or the like. The C source added is not limited, but anyC source may be used such as coal, coke, pitch coke, or plastic.

[0144] Furthermore, if these C sources are added, the same effects areproduced whether predetermined amounts of these C sources are mixedtogether before addition or are separately added to the hot metal. The Csource can be added whether it is massive, granular, or powdery.However, to easily dissolve in the hot metal, the C source is preferablycomposed of powders of grain size about 1 mm or smaller.

[0145] Method of Desulfurizing Hot Metal

[0146] A desulfurizing agent is applied to a hot metal desulfurizingprocess, the desulfurizing agent containing the recovered desulfurizingagent obtained as described above and the lime source and/or carbonsource added as required. The recovered desulfurizing agent can beapplied irrespective of the configuration of the hot metal desulfurizingfacility or the hot metal desulfurizing method.

[0147] The desulfurizing facility using the recovered desulfurizingagent may be based on mechanical agitation (KR method), the injectionmethod (torpedo), or a converter. The main chemical components of thehot metal are [mass % C]=3.5 to 5.0, [mass % Si]=0 to 0.3, [mass %S]=0.02 to 0.05, and [mass % P]=0.1 to 0.15. The temperature of the hotmetal is between 1,250 and 1,450° C. For a treatment, 5 to 300 tons ofhot metal is loaded into a refining container. To mix the desulfurizingagent into the hot metal, the recovered desulfurizing agent may be mixedwith a lime component before addition, or the recovered desulfurizingagent and the lime component may be cut out from separate hoppers beforeaddition. However, to ensure a certain degree of freedom for operations,it is effective to have a plurality of hoppers. With any refiningcontainer, the method used has only to allow the recovered desulfurizingagent and the lime component to be effectively supplied to a bathsurface. The amounts of recovered desulfurizing agent and lime componentintroduced are varied depending on the concentrations of Si, S, and P inthe hot metal. The total amount is desirably at most 20 kg/t.

[0148] Now, examples of the present invention will be described.

EXAMPLE 1 Example in which Desulfurization Slag Resulting from a HotMetal Pretreatment Based on the KR Method is Reused as a DesulfurizingAgent for a Hot Metal Desulfurizing Process Based on the KR Method

[0149] In this example, desulfurization slag resulting from adesulfurizing step based on the KR method is positively cooled andcrushed by the optimum treatment method, and is then reused as adesulfurizing agent for a desulfurizing process based on the KR method.For a recycling treatment, a mechanical crushing treatment, a naturalcooling treatment, and a watering treatment are properly combinedtogether. Specifically, the recycling treatment is executed by thefollowing method:

[0150] Desulfurization slag resulting from a hot metal pretreatmentbased on the KR method is first mechanically crushed while hot.Specifically, the desulfurization slag can be crushed using heavyequipment such as a shovel. Furthermore, the hot slag is watered tofacilitate cooling and disintegration. Specifically, a watering facilityis used for cooling.

[0151] Alternatively, the desulfurization slag may be cooled by leavingit as it is without watering. In this case, to facilitate cooling, thedesulfurization slag may be spread as thin as possible to increase thecontact area thereof which contacts with the atmosphere. Furthermore,reaction of the desulfurization slag with steam or carbon dioxide in theatmosphere may be promoted to facilitate disintegration of a limecomponent and generation of a compound such as calcium carbonate orcalcium hydroxide. Moreover, it is possible to crush the hot slag andthen pass it through a sieve to separate large masses such as metaltherefrom.

[0152] The desulfurization slag resulting from this method waspositively treated to facilitate cooling and crushing of thedesulfurization slag, thereby generating recovered desulfurizing agentgrains of maximum grain size 30 mm or smaller. The grains may further bemechanically crushed.

[0153] Since the recovered desulfurizing agent obtained has a maximumgrain size of 30 mm or smaller, a surface area is obtained which isenough to be involved in the desulfurization reaction.

[0154] Since the grains are appropriately fine, production of dust canbe prevented. Further, during reuse with the KR method, a decrease inyield caused by scattering can be prevented. Furthermore, the grains canbe properly caught in waves in a bath.

[0155] Moreover, generation of a compound such as calcium carbonate orcalcium hydroxide can be facilitated by positively promoting cooling anddisintegration as described above. When added to the hot metal, thesecompounds are decomposed while undergoing dehydrating and degassingreaction, to facilitate agitation of the hot metal. Further, thedecomposition increases the reaction surface area, thereby improving thedesulfurization efficiency.

[0156] The above recycling method efficiently provides a desulfurizingagent of grain size 30 mm or smaller having a new surface with adesulfurizing capability. This desulfurizing agent was used in theexample.

[0157] For comparison, a conventional desulfurizing agent was used whichcontained 90% of lime and about 5% of fluorite. Table 1 shows theaverage compositions of the conventional desulfurizing agent as acomparative example and the recovered desulfurizing agent as thisexample.

[0158] These desulfurizing agents were applied to a mechanical agitationtype desulfurizing apparatus under the conditions shown in Table 2 tocarry out a hot metal desulfurization pretreatment.

[0159] After the hot metal desulfurization, the desulfurization rateachieved by the mechanical agitation type desulfurizing apparatus waschecked for each of the desulfurizing agents. FIG. 3 shows therelationship between the amount of additives (fluxes) per unit amount ofhot metal and the desulfurization slag rate. In the graph of FIG. 3, acurve a indicates results for the desulfurization slag of the presentinvention, while a curve b indicates results for the conventionaldesulfurizing agent as a comparative example.

[0160] The graph of FIG. 3 indicates that with an equal amount ofdesulfurizing agent added per unit amount of hot metal, thedesulfurization rate of the recovered desulfurizing agent is about 50 to90% of that of the comparative desulfurizing agent. Furthermore, FIG. 4shows comparison based on the amount of lime component added per unitamount of hot metal. This graph indicates that in terms of a containedlime component, the recovered desulfurizing agent has a desulfurizingcapability substantially equivalent to that of the comparativedesulfurizing agent. Thus, a treatment with the recovered desulfurizingagent is expected to produce a desulfurizing effect equivalent to thatof a desulfurizing treatment with lime provided that the amount of limeadded per unit amount of hot metal, in the recovered desulfurizing agentadded, is estimated to be equivalent to the amount of lime required forthe desulfurization reaction.

[0161] Table 3 shows changes in the amount of lime used before and afterintroduction of the present process. The table indicates that the reuseof desulfurization slag reliably reduced the amount of lime used,thereby reducing treatment costs by about 40% compared to the costsrequired before introduction.

[0162] Table 4 shows changes in the amount slag generated before andafter introduction of the present process. The amount of slag generateddecreased by 3,000 t/month, thereby demonstrating that the presentprocess reduces not only desulfurization costs but also the amount ofslag to solve environmental problems.

[0163] This example produces significant effects. For example, itreduces the desulfurization costs, allows desulfurization slag to bereused, and reduces the amount of slag generated to solve environmentalproblems. Therefore, this example has a high industrial value.

EXAMPLE 2 Example Relating to the Desulfurization Rate of RecycledDesulfurization Slag and the Number of Times that the DesulfurizationSlag is Recycled

[0164] In this example, desulfurization slag resulting from adesulfurizing step was cooled and crushed by a natural cooling orwatering treatment and then reused as a desulfurizing agent during thesame process. A recovered desulfurizing agent of grain size 100 mm orsmaller and temperature 200° C. or lower was obtained without mechanicalcrushing, and was used as a desulfurizing agent according to thisexample. Furthermore, the recovered desulfurizing agent was used tocarry out desulfurization. The resultant slag was recovered again, andthe above method was used to set its size and temperature at 100 mm orsmaller and 200° C. or lower, respectively. Then, the slag was reused asa desulfurizing agent according to this example.

[0165] For comparison, the conventional desulfurizing agent was usedwhich contained 90% of lime and about 5% of fluorite. Table 1 shows theaverage compositions of the recovered desulfurizing agent andconventional desulfurizing agent used (all tables are shown at the endof the specification).

[0166] These desulfurizing agents were applied to the mechanicalagitation type desulfurizing apparatus under the conditions shown inTable 2 to carry out a hot metal desulfurization pretreatment.

[0167]FIG. 1 shows the relationship between the amount of lime added perunit amount of hot metal and the desulfurization rate at eachdesulfurization level. This figure indicates that the recovereddesulfurizing agents resulting from the first and second recyclingoperations had a desulfurizing capability equal to 80% or more of thatof the comparative desulfurizing agent. Thus, even with a treatmentusing the recovered desulfurizing agent resulting from the secondrecycling operation, properly increasing the amount of lime added isexpected to serve to produce a desulfurizing effect equivalent to thatof a desulfurizing treatment with lime.

[0168]FIG. 2 shows changes in the amount of desulfurizing agent usedbefore and after introduction of the present process, in connection withthe number of desulfurizing agent recycling operations and changes inthe amount of recovered desulfurizing agent used. The figure indicatesthat recycling the desulfurizing agent a number of times served todrastically reduce the amount of recovered desulfurizing agent used.When a recycling operation was preformed about three times, the amountof desulfurizing agent used decreased by about 75% compared to theamount measured before introduction. At the same time, the amount ofslag generated decreased drastically, thereby demonstrating that thepresent process is also effective on environmental problems.

[0169] Thus, according to this example, the use of the desulfurizingagent of the present invention reduces desulfurization costs, allowsdesulfurization slag to be recycled a number of times, and reduces theamount of waste to solve environmental problems.

EXAMPLE 3 Cooling and Crushing Based on a Watering Treatment

[0170] In this example, desulfurization slag resulting from adesulfurizing step is simultaneously cooled and crushed by a wateringtreatment, and is then dried and reused as a desulfurizing agent.Specifically, a watering facility is used to excessively water the hotslag subjected to the desulfurizing treatment until the slag iscompletely impregnated with water.

[0171] Subsequently, a drying apparatus is used to completely dry thewater-containing slag to obtain a desulfurizing agent of a sufficientlyreduced grain size of about 5 mm or smaller. Specifically, the apparatusused for drying may be a dryer, or a rotary kiln or the like may be usedto perform a large-scale drying operation. The size of the apparatus andthe like can be set depending on the required throughput or the like.Any apparatus and method may be used provided that water impregnatedinto the cooled slag can be sufficiently removed.

[0172] The thus recycled desulfurization slag was used as adesulfurizing agent according to this example.

[0173] For comparison, the conventional desulfurizing agent was usedwhich contained 90% of lime and about 5% of fluorite. Table 5 shows theaverage compositions of the recovered desulfurizing agent andconventional desulfurizing agent used.

[0174] These desulfurizing agents were applied to the mechanicalagitation type desulfurizing apparatus under the conditions shown inTable 2 to carry out a hot metal desulfurization pretreatment.

[0175]FIG. 5 shows the relationship between the amount of lime added perunit amount of hot metal and the desulfurization rate at eachdesulfurization level. This figure indicates that with an equal amountof lime introduced per unit amount of desulfurization slag, therecovered desulfurizing agent (indicated by a solid line) has adesulfurizing capability equal to about 70% of that of the comparativedesulfurizing agent (indicated by a broken line).

EXAMPLE 4 Cooling and Crushing Based on a Watering Treatment

[0176] In this example, desulfurization slag resulting from adesulfurizing step is simultaneously cooled and crushed by a wateringtreatment, and is then reused as a desulfurizing agent. That is, thewatering facility is used to uniformly water the hot slag resulting fromthe desulfurizing treatment, while using heavy equipment such as ashovel to agitate the slag. Specifically, the watering treatment iscontinued until the hot slag is cooled down to a temperature of 80 to150° C. Subsequently, the slag is left and cooled until its temperaturereaches the room temperature. Thus, a desulfurizing agent is obtainedwhich has a sufficiently reduced grain size of 5 mm or smaller. Thistarget cooling temperature is not limited to a certain value, but can beset depending on the required throughput or the like.

[0177] For comparison, the conventional desulfurizing agent was usedwhich contained 90% of lime and about 5% of fluorite. Table 7 shows theaverage compositions of the recovered desulfurizing agent andconventional desulfurizing agent used.

[0178] These desulfurizing agents were applied to the mechanicalagitation type desulfurizing apparatus under the conditions shown inTable 2 to carry out a hot metal desulfurization pretreatment.

[0179]FIG. 9 shows the relationship between the amount of lime added perunit amount of hot metal and the desulfurization rate at eachdesulfurization level. This figure indicates that with an equal amountof lime added per unit amount of hot metal, the desulfurizing agentrecovered by watering the slag until its temperature reaches 150° C. hasa desulfurizing capability substantially equivalent to that of thecomparative desulfurizing agent.

[0180] However, even with the same recovered desulfurizing agentsubjected to the watering treatment, the desulfurizing capability variesdepending on whether the watering treatment is stopped or continuedafter the temperature reaches 100° C. This is assumed to be because theamount of calcium hydroxide generated in the recovered desulfurizingagent increases significantly if the slag is cooled even after itstemperature reaches 100° C. and because such an excessive increase inthe amount of calcium hydroxide affects desulfurization, as shown inFIG. 7.

[0181] In this regard, FIG. 6 shows the relationship between thetemperature of the slag measured at the end of watering and the timerequired to cool 40 T of desulfurization slag down to the roomtemperature. This figure indicates that more time is required to coolthe slag down to the room temperature as the temperature of the slag ishigher at the end of watering.

[0182] The desulfurization treatment method of this example enablesdesulfurization slag to be efficiently treated, reduces the time andcost required to recover a desulfurizing agent, and allows a largeamount of desulfurization slag to be recycled to reduce the amount ofslag, thereby helping solve environmental problems.

EXAMPLE 5 Cooling and Crushing Based on a Natural Cooling Treatment

[0183] In this example, desulfurization slag resulting from adesulfurizing step is simultaneously cooled and crushed by a wateringtreatment, and is then reused as a desulfurizing agent. Specifically,the hot slag subjected to the desulfurization treatment is left as it isso as to maximize the contact area thereof which contacts with theatmosphere, and is then agitated using heavy equipment such as a shovel.Specifically, a recovered desulfurizing agent of temperature 200° C. orlower and a sufficiently reduced diameter can be obtained in three daysby spreading the hot slag so that its thickness is 0.5 m or smaller andagitating it about one to three times a day. The thickness duringcooling is not limited to this value, but the target thickness can beset depending on the required treatment time or throughput, the area ofa place that is available for the recycling treatment, or the like.Further, the agitation is carried out to increase the cooling speed, sothat its frequency may also be varied depending on the requiredtreatment time or throughput. The agitation may be omitted if muchtreatment time and throughput are available. This desulfurizing agentwas used as one according this example.

[0184] Furthermore, a recovered desulfurizing agent is similarlyobtained if the slag is left as it is without reducing its thickness.This is also shown as a desulfurizing agent according to this example.

[0185] Further, for comparison in terms of the cooling method,mechanically crushed desulfurization slag (a mechanically crushed agent)was used.

[0186] Furthermore, for comparison in terms of desulfurizing behavior,the conventional desulfurizing agent was used which contained 90% oflime and about 5% of fluorite. Table 8 shows the average compositions ofthe recovered desulfurizing agent and conventional desulfurizing agentused.

[0187] These desulfurizing agents were applied to the mechanicalagitation type desulfurizing apparatus under the conditions shown inTable 2 to carry out a hot metal desulfurization pretreatment.

[0188]FIG. 10 shows the relationship between the amount of lime addedper unit amount of hot metal and the desulfurization rate at eachdesulfurization level. This figure indicates that with an equal amountof lime added per unit amount of hot metal, the desulfurizing agentrecovered by executing natural cooling has a desulfurizing capabilitysubstantially equivalent to that of the comparative desulfurizing agent.

[0189] Further, even with the same non-watering treatment, thedesulfurization capability of the mechanically crushed recovereddesulfurizing agent is inferior to that of the recovered desulfurizingagent which has been naturally cooled. This is assumed to be because therecovered desulfurizing agent which has been naturally cooled containsseveral percent of calcium carbonate generated therein as shown in Table8 and because during the desulfurizing treatment, decomposition ofcalcium carbonate facilitates agitation of the hot metal and increasesthe reaction surface area to improve the desulfurization efficiency.

[0190] Comparison of the desulfurizing capability of the recovereddesulfurizing agent and the time required for the recycling treatmentwill be shown below in connection with the different methods ofrecovering desulfurization slag shown in Examples 3 to 5.

[0191] Even with the same recovered desulfurizing agent, a higherdesulfurization capability is obtained by crushing slag by watering (at150° C.) than by mechanically crushing it. This is assumed to be becausethe recovered desulfurizing agent crushed by watering contains severalpercent of calcium carbonate generated therein and because during thedesulfurizing treatment, decomposition of calcium carbonate facilitatesagitation of the hot metal and increases the reaction surface area toimprove the desulfurization efficiency.

[0192] However, with the same recovered desulfurizing agent subjected tothe watering treatment, the desulfurization capability varies dependingon whether the watering operation is controlled (150° C.) or iscontinued until the desulfurizing agent is completely impregnated withwater. This is assumed to be because cooling the desulfurization slagdown to a temperature of 100° C. or lower significantly increases theamount of calcium hydroxide (Ca(OH)₂) generated in the recovereddesulfurization slag and because such an excessive increase in theamount of calcium hydroxide affects desulfurization.

[0193] Further, Table 6 shows a comparison of the treatment conditionsfor the desulfurization slag and the time required to treat about 40 Tof desulfurization slag. This table indicates that the watereddesulfurization slag is cooled much faster than that left as it iswithout undergoing watering. The table also indicates that thewatering-based treatment allowed the slag to be perfectly crushed withinthe time required for cooling. Furthermore, controlling the amount ofwater supplied eliminates the need for a drying treatment after thewatering treatment, thereby saving the facilities, cost, and timerequired for crushing and drying. Further, watering during the treatmentprevents production of dust from the desulfurization slag being treated.

[0194] On the other hand, with the natural cooling treatment for theslag without watering, the required treatment time is 170 hours if thethickness of the desulfurization slag is 1.5 m but decreasesdramatically down to 70 hours if the thickness is 0.4 m. In this regard,FIG. 8 shows the relationship between the treatment time and thetemperature for the different thicknesses for the respective treatments(the solid line indicates a thickness of 0.4 m, while the broken lineindicates a thickness of 1.5 m). The treatment speed increases withdecreasing thickness. However, the thickness is determined on the basisof the required cooling speed, the area of a place available forcooling, or the like. The non-watering treatment method according to thepresent invention requires slightly more time than the wateringtreatment, but does not require any watering facilities. Further, withthe watering treatment, it is difficult to uniformly water the slag inthe case of bulk treatment, possibly resulting in water-containing slag.It is thus difficult to provide such control that such slag is notgenerated. Problems with the water-containing slag are that it isdifficult to handle after the recovery treatment, that flame isgenerated when it is introduced, that it has an inadequate desulfurizingcapability, and the like. On the other hand, the non-watering naturalcooling treatment method allows the slag to be easily uniformly treatedeven in the case of bulk treatment. This method also causes the limecomponent in the slag to be “powdered” (part of the lime component inthe slag reacts with moisture or carbon dioxide in the atmosphere duringthe natural cooling operation to change the volume of the slag, whichthus becomes powdery), so that the grain size of the slag decreases asthe time elapses. This eliminates the need for mechanical crushing afterthe cooling treatment. Furthermore, part of the lime component in theslag reacts with carbon dioxide in the atmosphere to generate calciumcarbonate, which is effective in improving the desulfurization reactionefficiency.

[0195] As described above, by effectively using the desulfurization slagtreatment method of this example according to the conditions such as theamount of desulfurization slag to be recovered and the availablefacilities, the desulfurization slag can be inexpensively andefficiently treated to reduce the time and cost required to recover thedesulfurization slag. This in turn allows a large amount ofdesulfurization slag to be recycled and drastically reduces the amountof slag to solve environmental problems.

EXAMPLE 6 Screening Desulfurization Slag After Separation of Metal

[0196] In this example, while hot at 900 to 1,200° C., desulfurizationslag resulting from a desulfurizing step is screened using a screeningapparatus 12 with a ¤70-mm mesh, and is thus separated intodesulfurization slag of a smaller diameter containing a large amount ofunreacted lime and metal of a larger diameter. In this case, any sievecan be used without creating any problems provided that it can beoperated over a temperature range from 900 to 1,200° C.; a sieve made ofiron is sufficient. The shape, specification, and the like of this sieveare not limited provided that it can be operated in a desulfurizationslag treatment field. Further, the mesh of the sieve is restricted bythe supply apparatus in the desulfurizing facility where the slag isused as a desulfurizing agent. An appropriate mesh can be used withoutcreating any problems.

[0197] The sieved desulfurization slag of a small diameter is cooled andthen transported, as a recovered desulfurizing agent, to the supplyapparatus on the desulfurizing facility, where it is used.

[0198] On the other hand, the residual metal can be easily recoveredusing the above described method. Further, this metal can then be reusedas an iron source for the hot metal pretreatment step, therebysignificantly contributing to increasing the yield of iron.

[0199] In this example, desulfurization slag was recovered underconditions such as those shown in Table 9. The sieve was angularlyinstalled relative to a horizontal surface. Hot slag was screened bydropping it from the top of the sieve. Table 10 shows the mass balanceof a desulfurizing agent during its recovery process. It has beenconfirmed that even if the desulfurization slag resulting from thedesulfurizing step is sieved, about 90% of CaO in the desulfurizationslag is recovered, with the metal component effectively removed. Thatis, the method of the present invention has proved to enable the slagcomponent in the desulfurization slag to be effectively recoveredwithout magnetic separation.

[0200] The recovered desulfurizing agent and the lime were applied tothe desulfurizing treatment, and desulfurizing behavior was checked. Forsome levels, the mixture of the recovered desulfurizing agent with thelime was carried out in two manners. That is, these components weremixed together and then loaded into a hopper and then a certain part ofthe mixture was cut out, or a recovered desulfurizing agent and a limecomponent were cut out from the respective hoppers. Table 17 also showsthe average composition of a lime-based desulfurizing agent as aconventional example. These desulfurizing agents were applied to themechanical agitation type desulfurizing apparatus under the testconditions shown in Table 11 to carry out hot metal desulfurization.

[0201]FIG. 16 shows the relationship between the amount of lime addedper unit amount of hot metal and the desulfurization rate. Theconcentration of the lime component in the recovered desulfurizationslag is lower than that of the comparative example, so that the isamount of desulfurizing agent introduced is larger in this example thanin the comparative example. However, the desulfurizing capability ofthis example is equivalent to that of the comparative example, therebydemonstrating the effectiveness of the present invention.

[0202] Then, to examine the effects of the temperature of thedesulfurization slag on dust, the hot slag was left as it was for 30minutes to 4 hours and was then passed through the screening apparatus12, previously described. Then, it was checked how dust was produced andhow the temperature changed. Table 14 shows how dust was produced whenthe slag was screened with its temperature changed. The table indicatesthat production of dust changed significantly after pre-screeningtemperature reached 600° C. This nature can be utilized to perform ascreening operation or the like without any dust collectors. Further,Table 15 shows the result of measurement of a decrease in temperatureduring the screening operation. During screening, the temperaturedecreases by about 100° C., thus reducing the time required for cooling.Further, Table 16 shows how dust was produced in the screening facilityaccording to the present invention and in a screening facility that doesnot have the diagonal plate (14) or sliding way (16), shown in FIG. 12.

[0203] With the screening operation according to the present invention,even desulfurization slag that is likely to produce dust at atemperature of 600° C. or lower can be screened without any dustcollectors. The method of this example significantly simplify a facilitywhich is used to recycle desulfurization slag and which adjusts thegrain size of dry desulfurization slag. Further, production of dust canbe prevented during screening using a simple facility. Furthermore,during screening, the slag efficiently contacts with the atmosphere andis thus efficiently cooled. Moreover, by loading the desulfurizationslag onto a dump truck using the loading method described herein, theloading operation can be achieved while preventing production of dustwithout any dust collectors.

[0204] The treatment of this example enables the desulfurizing agent tobe inexpensively and efficiently treated and reduces the amount ofdesulfurization slag to be treated compared to the prior art, therebydrastically reducing costs. Other significant effects of this exampleare the possibility of recovering and recycling a large amount ofdesulfurization slag and a decrease in the amount of slag, which leadsto solution of environmental problems.

EXAMPLE 7 Handling of a Recovered Desulfurizing Agent

[0205] If a recovered dry desulfurizing agent is loaded onto a dumptruck for recycling, then naturally, it is powdered faster and is verylikely to produce dust. However, the amount of dust produced can beminimized by using a vehicle with a sucking capability instead of thedump truck to transport the recovered desulfurizing agent. Further, therecovered desulfurizing agent can concurrently be screened by installinga mesh at a suction port during suction, thereby enabling efficienthandling and transportation with a reduced amount of dust produced.

[0206] Specifically, a suction hose is connected to the vehicle with thesucking capability to suck and load a recovered desulfurizing agentobtained by cooling and crushing desulfurization slag resulting from thehot metal desulfurizing step. At this time, by installing a jig having amesh for screening at the suction port, the desulfurizing agent canconcurrently be screened so that the required grain size can beobtained. In this case, the jig attached to the suction port duringsuction may be selected on the basis of the required level of the sieve,i.e. the desired grain size of the recovered desulfurizing agent.

[0207] Furthermore, if no screening operation is required duringsuction, the mesh need not be installed at the suction port, and thedesulfurizing agent may be sucked using only the hose.

[0208] Moreover, if only large masses of a several tens of cm level mustbe removed, the recovered desulfurization slag was simultaneously loadedand sieved on the basis of the sucking operation using a simple jig onlyhaving partitions arranged at the tip thereof and formed of metal barsas shown in FIG. 11. In this example, desulfurization slag resultingfrom the mechanical agitation type desulfurizing step was used. Thedesulfurization slag was left to be naturally cooled in a building forabout 6 days. During this period, to facilitate cooling, thedesulfurization slag was agitated about three times a day.

[0209] The hose used for suction had a diameter of 15 cm, and the jig 11attached to the suction port during suction included two nets with a 30-and 5-mm meshes and a conical jig. These jigs all had an air suctionport. Furthermore, as a comparative example, the desulfurization slagwas sucked using only the hose and without any jigs. Further, Table 12shows the specification of the suction vehicle used.

[0210] Table 13 shows throughput and suction performance as results forsuction of a recovered desulfurizing agent using the various types ofsuction ports previously described. These results indicate that therecovered desulfurizing agent can be efficiently loaded at a rate ofabout 1 t/min. while being simultaneously sieved, whichever suction portis used. Further, it has been confirmed that during suction, thetemperature of the recovered desulfurizing agent decreases by about 30°C. after the treatment as a result of the positive contact of thedesulfurizing agent with the atmosphere.

[0211] Furthermore, most of the sieved large masses and the slag on thesieve are metal and can thus be used for another process.

[0212] It has been ascertained that with the handling method of thepresent invention, even a recovered desulfurizing agent which iscomposed of fine powders and which is likely to produce dust can besimultaneously and efficiently sieved and loaded onto a transportvehicle without using any dust collectors.

[0213] In this example, with the desulfurizing agent treating methodused to recycle desulfurization slag, a transport vehicle with a suckingcapability can be used to load a recovered desulfurizing agent whileminimizing the amount of dust produced. Furthermore, the recovereddesulfurizing agent can be efficiently treated by simultaneouslyperforming a sucking and screening operations. Moreover, the recovereddesulfurizing agent efficiently contacts with the atmosphere or suctionair and is thus efficiently cooled.

EXAMPLE 8 Measurement of the Content of CaO in Desulfurization Slag

[0214] In this example, desulfurization slag resulting from a mechanicalagitation type desulfurizing step was used. The desulfurization slag waspassed through the screening apparatus composed of a net with a ¤70-mmmesh.

[0215] The following are the results of the reuse, in a mechanicalagitation type desulfurizing facility, of the recovered desulfurizingagent having its grain size adjusted. FIG. 14 shows the relationshipbetween the amount of pure CaO component and the amount of materialdesulfurized ΔS (the amount of sulfur before treatment (S)—the amount ofsulfur after treatment (S)). It has been found that the amount of pureCaO component has a clear correlationship with the amount of materialdesulfurized. This nature can be utilized to easily determine the amountof desulfurizing agent introduced depending on the amount of sulfurbefore treatment (S), thereby achieving a stable desulfurization rate.

[0216] Further, FIG. 15 shows the relationship between the bulk densityand the CaO mass %. This figure indicates that the bulk densitydecreases with increasing amount of CaO. This tendency can be utilizedto estimate the CaO mass % of the recovered desulfurizing agent bymeasuring the bulk density thereof.

[0217] According to this example, when desulfurization slag was reusedin the mechanical agitation type desulfurizing facility, the requiredamount of desulfurizing agent could be determined promptly, whileachieving a stable desulfurization rate.

EXAMPLE 9 Example Relating to Improvement of Desulfurization Efficiencyby Addition of a Lime Source (CaO, CaCO₃, Ca(OH)₂)

[0218] Desulfurization slag resulting from a desulfurizing step wascooled and crushed by a natural cooling or watering treatment to obtaina desulfurizing agent (hereinafter referred to as a “recovereddesulfurizing agent”). One or more of the above-described desulfurizingagents were added to the recovered desulfurizing agent, which was thenused as a desulfurizing agent according to this example. In acomparative example, only the recovered desulfurizing agent was used.Table 18 shows the amount of recovered desulfurizing agent added, thetypes of desulfurizing agents, and the amounts of desulfurizing agentsadded.

[0219] These desulfurizing agents were applied to the mechanicalagitation type desulfurizing apparatus under the test conditions shownin Table 19 to carry out hot metal desulfurization.

[0220]FIG. 19 shows the relationship between the amount of limecomponent added per unit amount of hot metal and the desulfurizationrate at each desulfurization level. This figure indicates that with anequal amount of lime added per unit amount of hot metal, the use of the(d) recovered desulfurizing agent+lime slightly increases thedesulfurization rate compared to the use of (a) only the recovereddesulfurizing agent. The FIG. also indicates that the use of the (b)recovered desulfurizing agent+CaCO₃, the (c) recovered desulfurizingagent+CaCO₃+Ca(OH)₂, or the (e) recovered desulfurizingagent+lime+CaCO₃+Ca(OH)₂ increases both the amount of CaCO₃ and Ca(OH)₂added and the desulfurization rate compared to the use of (a) only therecovered desulfurizing agent. This is assumed to be because during thedesulfurizing treatment, decomposition of CaCO₃ or Ca(OH)₂ increases thereaction surface area or facilitates agitation of hot metal to improvethe desulfurization efficiency, as previously described.

[0221] Table 20 shows the ratio of CaO to the recovered desulfurizingagent, the amount of recovered desulfurizing agent added, the types ofdesulfurizing agents, the amounts of desulfurizing agents added, and thetotal amount of desulfurizing agents which are measured if 8 kg-CaO/T oflime component is contained in the desulfurizing agents of thecompositions used in this example. If each desulfurizing agent requiresthe same amount of lime component, when only the recovered desulfurizingagent is used, a large amount of desulfurizing agent is required if therecovered desulfurizing agent contains only a small amount of pure limecomponent. However, addition of lime and calcium carbonate or calciumhydroxide improves the desulfurization slag rate and reduces the amountof desulfurizing agent used. This reduces the amount of slag dischargedafter the treatment. Furthermore, the compositions of the desulfurizingagents can be freely adjusted according to the state of the hot metal tobe treated or the required treatment conditions. Moreover, similareffects are produced even if these desulfurizing agents are separatelyadded to the hot metal instead of being mixed together.

EXAMPLE 10 Example Relating to Improvement of Desulfurization Efficiencyby Addition of Various Carbon Sources

[0222] Desulfurization slag resulting from a desulfurizing step wascooled and crushed by a natural cooling or watering treatment to obtaina desulfurizing agent (hereinafter referred to as a “recovereddesulfurizing agent”). Various C sources were added to the recovereddesulfurizing agent, which was then used as a desulfurizing agentaccording to this example. Table 21 shows the amount of recovereddesulfurizing agent added, the types of desulfurizing agents, and theamounts of desulfurizing agents added. The C sources were composed ofpowders of grain size 1 mm or smaller.

[0223] These desulfurizing agents were applied to the mechanicalagitation type desulfurizing apparatus under the test conditions shownin Table 22 to carry out hot metal desulfurization.

[0224]FIG. 20 shows the relationship between the amount of limecomponent added per unit amount of hot metal and the desulfurizationrate at each desulfurization level. This figure indicates that with anequal amount of lime added per unit amount of hot metal, the use of the(d) to (d) recovered desulfurizing agent+C source increases thedesulfurization rate linearly with the amount of C source added,compared to the use of (a) only the recovered desulfurizing agent. Ithas also been confirmed that equivalent effects are produced regardlessof the type of the C source or the method of adding the C source. Thisis assumed to be because the C source acts as a reducing agent duringthe desulfurizing step to improve the desulfurization rate, aspreviously described. Further, part of the C source dissolved in the hotmetal to increase the concentration of C in the hot metal by 0.1 to 0.5%in each case under the conditions of this example.

[0225] By thus adding a C source such as anthracite or coke, thedesulfurization rate can be improved while reducing the amount of slagdischarged after the treatment. Any C source may be used, such as coal,coke, pitch coke, or plastic. Furthermore, the compositions of thedesulfurizing agents can be freely adjusted according to the state ofthe hot metal to be treated or the required treatment conditions.Moreover, similar effects are produced even if these desulfurizingagents are separately added to the hot metal instead of being mixedtogether.

[0226] As described above, the present invention provides a method ofdesulfurizing hot metal, which method effectively reuses desulfurizationslag resulting from a hot metal desulfurizing treatment in order toreduce the costs of hot metal desulfurization and the amount of slaggenerated. Further, the present invention provides a desulfurizing agentthat allows a hot metal desulfurizing treatment with a reduced amount ofslag generated to be inexpensively carried out. The present inventionproduces significant effects. For example, it reduces desulfurizationcosts, allows desulfurization slag to be recycled, and reduces theamount of slag to solve environmental problems. Therefore, the presentinvention has a high industrial value. TABLE 1 Main components ofaverage conventional desulfurizing agent and recovered desulfurizingagent Main component T-Fe CaO SiO₂ Al₂O₃ T-S CaF₂ (1) Conventional —90.0 — — — 5.0 desulfurizing agent (2) Recovered 19.2 53.6  9.3  4.1 1.72.1 desulfurizing agent (3) Desulfurization slag 19.7 45.0 12.0 11.0 3.22.2 resulting from recovered desulfurizing agent-used defulfurizingtreatment

[0227] TABLE 2 Test conditions Item Contents Desulfurizing methodMechanically agitating type desulfurizing apparatus Treatment ContainerHot metal ladle Throughput   140-160t Hot metal temperature  1300-1450°C. Hot metal [S]  0.05-0.02% Amount of 0 to 30 kg - CaO/t desulfurizingagent

[0228] TABLE 3 KR desulfurization treatment conditions After Beforeintroduction introduction of recycling of recycling Item process processHot metal [S] 0.05-0.02% 0.05-0.02% Amount of 8 kg/t   3-5 kg/t limeused Amount of —   8 kg - CaO/t desulfurizing agent used

[0229] TABLE 4 Changes in amount of slag generated associated withintroduction of desulfurization recycling process Amount of slaggenerated (t/month) Before recycling 8000 After recycling 4000 Amount ofdecrease in 4000 amount of slag generated

[0230] TABLE 5 Main components of average conventional desulfurizingagent and recovered desulfurizing agent Main Conventional Drying aftercomponent desulfurizing agent watering CaO 90.0 53.6 T-Fe — 19.2 SiO₂ —9.3 Al₂O₃ — 4.1 T-S — 1.7 CaF₂  5.0 2.1 CaCO₃ — 2.3 Ca(OH)₂ — 18.3(Unit: mass %)

[0231] TABLE 6 Time required for desulfurizing treatment method and 40Ttreatment Non-watering Watering Natural cooling Drying DesulfurizationDesulfurization Treatment Watering after slag thickness slag thicknessMechanical method 150° C. 80° C. watering 1.5 m 0.4 m crushing Amount ofwater  8 15 30 — — supplied (t) Mechanical Not required Not Not requiredRequired crushing required Drying Not required Required Not required Notrequired Time required 30 24 10 170 70 50 to cool slag down to 100° C.

[0232] TABLE 7 Main components of average conventional desulfurizingagent and recovered desulfurizing agent Conventional watered wateredMain desulfurizing agent agent component agent (150° C.) (80° C.) CaO90.0 53.6 53.6 T-Fe — 19.2 19.2 SiO — 9.3 9.3 Al₂O₃ — 4.1 4.1 T-S — 1.71.7 CaF₂ 5.0 2.1 2.1 CaCO₃ — 5.2 1.1 Ca(OH)₂ — 3.3 24.1 (Unit: mass %)

[0233] TABLE 8 Main components of average conventional desulfurizingagent and recovered desulfurizing agent Non- watered and Non-wateredConventional naturally mechanically Main desulfurizing cooled crushedcomponent agent agent agent CaO 90.0 53.6 53.6 T-Fe — 19.2 19.2 SiO₂ —9.3 9.3 Al₂O₃ — 4.1 4.1 T-S — 1.7 1.7 CaF₂  5.0 2.1 2.1 CaCO₃ — 6.5 0.4Ca(OH)₂ — 2.5 0.5 (Unit: mass %)

[0234] TABLE 9 Recovery treatment conditions for desulfurization slagDesulfurization 600-1200° C. slag temperature Sieve Made of iron, mesh70 mm × 70 mm Sieve angle  8-15° Throughput 2t/times

[0235] TABLE 10 Changes in composition during recovery processConventional Hot slag Recovered desulfurizing (before desulfurizingagent screening) agent Weight Metal component % 0 20 7.2 ratio Slagcomponent % 100 80 92.8 Slag CaO 90 54 53.8 compo- T-Fe — 18.5 18.2sition SiO₂ — 9.0 8.8 Al₂O₃ — 4.1 4.1 T-S — 1.7 1.8 CaF₂ 5 2.1 1.8 CaCO₃— 0.4 1.3 Ca(OH)₂ — 0.5 1.0 (unit: mass %)

[0236] TABLE 11 Test conditions Item Contents Desulfurizing Mechanicallyagitating method type desulfurizing apparatus Refining Hot metal ladlecontainer Throughput   130-165t Hot metal  1300-1450° C. temperatureInitial [S] in  0.02-0.05% hot metal Agitation time 5 to 7 minutesAmount of    6-30 kg/t desulfurizing agent

[0237] TABLE 12 Specification of suction vehicle Suction air quantity 80m³ Vacuum pressure −650 mmHg Suction tank capacity 5.5 m³ Connectionhose diameter 15 cm

[0238] TABLE 13 Sucking method and Recovered desulfurizing agent loadingcapability Sucking method Amount of recovered Suction Sucking Diameterof Suction desulfurizing agent time capability hose used port typesucked (t) (min.) (t/min.) 15 cm  5 mm mesh 14.2 14.7 1.00 15 cm 30 mmmesh 22.4 20.13 1.11 15 cm Cone-shaped 6.00 5.10 1.18 15 cm Cone-shaped16.2 16.2 1.00 15 cm Cone-shaped 11.1 12.5 0.89 15 cm Cone-shaped 29.733.2 0.89 15 cm Cone-shaped 15.0 15.9 0.94 15 cm Only hose 13.8 9.8 1.40

[0239] TABLE 14 Screening temperature and dust generation Temperaturebefore screening Dust generation 420° C. Large 610° C. Small 940° C.Very little

[0240] TABLE 15 Decrease in temperature during screening Before AfterDecrease in screening screening temperature 1 930° C. 790° C. Δ140° C. 2 940° C. 880° C. Δ60° C. 3 940° C. 860° C. Δ80° C.

[0241] TABLE 16 Dust generation observed when present facility is usedSlag Dust temperature generation Present screening 450° C. Littlefacility Screening facility 480° C. Very large without diagonal plate orsliding way

[0242] TABLE 17 Average desulfurizing agent composition used when bothrecovered desulfuring agent and lime are used Recovered desulfurizingagent + 50% lime Conventional Recovered Recovered (introduced Recovereddesulfurizing Recovered desulfurizing desulfurizing throughdesulfurizing agent desulfurizing agent + agent + different agent +(Comparative agent 25% lime 50% lime system) 80% lime example) (Examplea) (Example b) (Example c) (Example d) (Example e) Slag composition %CaO 90 53.8 65.4 75.8 75.8 90.1  T-Fe — 18.2 14.0 9.3 9.3 4.2 SiO₂ — 8.86.8 4.6 4.6 2.1 Al₂O₃ — 4.1 3.2 2.2 2.2 1.0 T-S — 1.8 1.4 1.0 1.2 0.5CaF₂  5 1.8 1.4 1.1 1.2 0.6 CaCO₃ — 1.3 1.1 0.7 0.8 0.6 Ca(OH)₂ — 1.00.8 0.6 0.7 0.6

[0243] TABLE 18 Average desulfurizing agent composition used when bothrecovered desulfurizing agent and lime source Recovered desulfurizingExample agent Lime CaCO₃ Ca(OH)₂ a 100 — — — b 70 — 30 — c 60 — 20 20 d50 50 — — e 30 50 10 10 (Unit: mass %)

[0244] TABLE 19 Test conditions Item Contents Desulfurizing Mechanicallyagitating type method desulfurizing apparatus Treatment Hot metal ladlecontainer Throughput   140-160t Hot metal  1300-1450° C. temperature Hotmetal [S]  0.05-0.02%

[0245] TABLE 20 Amounts of recovered desulfurizing agent and lime sourceadded when 8.0 kg/T of CaO component is required in desulfurizing agentRecovered desulfurizing agent Added desulfurizing agent Total CaO Amountof Ca(OH)₂ amount component recovered Lime CaCO₃ Amount of of indesulfurizing CaO Amount of Amount of Ca(OH)₂ desulfurizingdesulfurizing agent added component lime added CaCO₃ added added agentagent wt % kg/T wt % wt % kg/T wt % kg/T wt % kg/T kg/T kg/T 100  11.470 — — — — — — 11.4 8.0 20.0 40 — — — — — — 20.0 70 12.5 40 — — 30 5.4 —— 17.9 60 9.5 40 — — 20 3.2 20 3.2 15.9 50 5.7 40 50 5.7 — — — — 11.4 303.2 40 50 5.3 10 1.1 10 1.1 10.6

[0246] TABLE 21 Average desulfurizing agent composition used when bothrecovered desulfurizing agent and C source are used C source AmountRecovered of C desulfurizing source Adding agent Type added methodExample 100 — 0 — a 95 Coke 5 Mixture b 95 Anthracite 5 Mixture c 80Coke 20 Mixture d 70 Coke 30 Separate addition (Unit: mass %)

[0247] TABLE 22 Test conditions Item Contents Desulfurizing Mechanicallyagitating method type desulfurizing apparatus Treatment Hot metal ladlecontainer Throughput   140-160t Hot metal  1300-1450° C. temperature Hotmetal [S]  0.05-0.02%

What is claimed is:
 1. A method of manufacturing a hot metaldesulfurizing agent, the method being comprising executing a treatmentfor creating a new surface in desulfurization slag resulting from amechanical agitation type hot metal desulfurizing treatment.
 2. A methodof manufacturing a hot metal desulfurizing agent for use in a mechanicalagitation type hot metal desulfurizing treatment according to claim 1,the method being comprising executing a treatment for creating a newsurface in desulfurization slag resulting from the mechanical agitationtype hot metal desulfurizing treatment.
 3. A method according to claim1, comprising a step of providing desulfurization slag resulting fromthe mechanical agitation type hot metal desulfurizing treatment and astep of executing the treatment for creating a new surface in theprovided desulfurization slag.
 4. A method according to claim 1, whereinthe step of executing the treatment for creating a new surface includescrushing a desulfurization slag grain and/or separating an aggregate ofa plurality of desulfurization slag grains into desulfurization slaggrains.
 5. A method according to claim 1, wherein the step of executingthe treatment for creating a new surface includes crushing thedesulfurization slag grain and/or separating the aggregate of aplurality of desulfurization slag grains into desulfurization slaggrains by air-cooling the desulfurization slag and/or applyingmechanical energy to the desulfurization slag.
 6. A method according toclaim 5, wherein the desulfurization slag is cooled using one or twomethods selected from the group consisting of air cooling and watercooling.
 7. A method according to claim 6, wherein the air cooling iscarried out using one or two methods selected from the group consistingof natural air cooling and forced air cooling.
 8. A method according toclaim 6, wherein the step of executing the treatment for creating a newsurface by water cooling comprises a step of watering thedesulfurization slag, and this watering step controls the amount ofwater provided so as to maintain the temperature of the desulfurizationslag at 100° C. or higher at the end of the watering, so that only bymeans of cooling based on the watering, the aggregate of desulfurizationslag can be separated into desulfurization slag grains and/or thedesulfurization slag grains can be crushed.
 9. A method according toclaim 1, wherein the step of executing the treatment for creating a newsurface comprises a step of water-cooling the desulfurization slag and astep of drying a recovered desulfurizing agent resulting from the watercooling.
 10. A method according to claim 1, wherein the step ofexecuting the treatment for creating a new surface comprises a step ofcooling the desulfurization slag and a step of adjusting the grain sizesof the desulfurization slag and the recovered desulfurizing agent.
 11. Amethod according to claim 10, wherein the step of adjusting the grainsize of the recovered desulfurizing agent using a sieve is executed at atemperature of 600° C. or higher.
 12. A method according to claim 1,wherein the step of executing the treatment for creating a new surfacecomprises a step of executing at least one or two treatments selectedfrom the group consisting of a treatment for magnetically separating andremoving metal from the desulfurization slag or the recovereddesulfurizing agent, a treatment for removing large masses from thedesulfurization slag or the recovered desulfurizing agent to set thegrain size to 100 mm or smaller, and a treatment for setting thetemperature of the desulfurization slag or the recovered desulfurizingagent to 200° C. or lower.
 13. A method according to claim 1, whereinthe step of executing the treatment for creating a new surface comprisesa step of setting the grain size of the desulfurization slag to 100 mmor smaller and the temperature thereof to 200° C. or lower.
 14. A methodof transporting a recovered desulfurizing agent, the method beingcomprising a step of loading a recovered desulfurizing agent resultingfrom a mechanical agitation type hot metal desulfurizing treatment ontoa transport vehicle using a pair of movable basket sections that can beopened and closed and a step of using the transport vehicle to transportthe recovered desulfurizing agent to a desulfurizing treatment facility.15. A method according to claim 14, comprising a step of screening thecooled and crushed recovered desulfurizing agent to remove large massesbefore or simultaneously with the step of using the transport vehicle totransport the recovered desulfurizing agent.
 16. A method oftransporting a recovered desulfurizing agent, the method beingcomprising a step of loading a recovered desulfurizing agent resultingfrom a mechanical agitation type hot metal desulfurizing treatment ontoa transport vehicle having a capability of sucking the recovereddesulfurizing agent and a step of using the transport vehicle totransport the recovered desulfurizing agent to a desulfurizing treatmentfacility.
 17. A method according to claim 14, wherein the step ofloading the recovered desulfurizing agent is executed while adjustingthe height from which the recovered desulfurizing agent is dropped ontothe transport vehicle, to 1.5 m or less.
 18. An apparatus which screensa recovered desulfurizing agent, the apparatus being comprising anapparatus main body having a sieve mesh that screens a crushed recovereddesulfurizing agent and an air sucking hose attached to the apparatusmain body to facilitate suction of the recovered desulfurizing agentinto the apparatus main body.
 19. An apparatus which screens a recovereddesulfurizing agent, the apparatus being comprising a member having adiagonally arranged sieve mesh, a diagonal plate diagonally arrangedbelow the member and on which a minus sieve slides down, and a slidingway on which the minus sieve from the diagonal plate falls and thenslides down, and in that the apparatus is arranged so that the spacingbetween the member having the sieve mesh and the diagonal plate and thevertical height of drop of a linkage between the diagonal plate and thesliding way are each 500 mm or less and the height of drop from thesliding way to the ground surface is 500 mm or less.
 20. A hot metaldesulfurizing agent comprising a recovered desulfurizing agent resultingfrom a mechanical agitation type hot metal desulfurizing treatment. 21.A hot metal desulfurizing agent according to claim 20, comprising therecovered desulfurizing agent resulting from the mechanical agitationtype hot metal desulfurizing treatment, the hot metal desulfurizingagent being used for the mechanical agitation type hot metaldesulfurizing treatment.
 22. A hot metal desulfurizing agent accordingto claim 20, comprising the recovered desulfurizing agent resulting fromthe mechanical agitation type hot metal desulfurizing treatment andhaving a new surface created therein.
 23. A hot metal desulfurizingagent according to claim 20, comprising the recovered desulfurizingagent which results from the mechanical agitation type hot metaldesulfurizing treatment and in which part or all of an aggregate ofrecovered desulfurizing agent grains is separated into pieces.
 24. A hotmetal desulfurizing agent according to claim 20, having a maximum grainsize of 100 mm or smaller.
 25. A hot metal desulfurizing agent accordingto claim 20, comprising the recovered desulfurizing agent resulting fromthe mechanical agitation type hot metal desulfurizing treatment, the hotmetal desulfurizing agent further containing one or two sources selectedfrom the group consisting of a lime source and a carbon source.
 26. Ahot metal desulfurizing agent according to claim 20, wherein the one ortwo sources selected from the group consisting of the lime source andthe carbon source are mixed with the recovered desulfurizing agentresulting from the mechanical agitation type hot metal desulfurizingtreatment, and this mixture is added to hot metal.
 27. A hot metaldesulfurizing agent according to claim 20, wherein the one or twosources selected from the group consisting of the lime source and thecarbon source are separated from the recovered desulfurizing agentresulting from the mechanical agitation type hot metal desulfurizingtreatment, and the one or more sources and the recovered desulfurizingagent are separately added to hot metal.
 28. A hot metal desulfurizingagent according to claim 20, wherein the lime source is at least one ortwo sources selected from the group consisting of lime, calciumcarbonate, and calcium hydroxide.
 29. A hot metal desulfurizing agentaccording to claim 20, wherein the total amount of calcium carbonate andcalcium hydroxide, which are included in the group of lime sources,corresponds to 40 wt % or less of the whole hot metal desulfurizingagent.
 30. A hot metal desulfurizing agent according to claim 20,wherein the amount of carbon source corresponds to 30 wt % or less ofthe whole hot metal desulfurizing agent.
 31. A hot metal desulfurizingagent according to claim 20, wherein the carbon source is powders ofgrain size 1 mm or smaller.
 32. A hot metal desulfurizing agentaccording to claim 20, wherein the carbon source is at least one or twosources selected from the group consisting of coal, coke, and pitch. 33.A method of manufacturing low-sulfur hot metal, the method beingcomprising desulfurizing hot metal by adding, to the hot metal, a hotmetal desulfurizing agent comprising a recovered desulfurizing agentresulting from a mechanical agitation type hot metal desulfurizingtreatment.
 34. A method of manufacturing low-sulfur hot metal, themethod being comprising desulfurizing hot metal using a mechanicalagitation type hot metal desulfurizing treatment by adding, to the hotmetal, a hot metal desulfurizing agent comprising a recovereddesulfurizing agent resulting from the mechanical agitation type hotmetal desulfurizing treatment.
 35. A method of manufacturing low-sulfurhot metal according to claim 33, the method being comprisingdesulfurizing the hot metal by adding, to the hot metal, thedesulfurizing agent comprising the recovered desulfurizing agentresulting from the mechanical agitation type hot metal desulfurizingtreatment and having a new-surface created therein.
 36. A method ofmanufacturing low-sulfur hot metal according to claim 33, the methodbeing comprising desulfurizing the hot metal by adding, to the hotmetal, the desulfurizing agent comprising the recovered desulfurizingagent which results from the mechanical agitation type hot metaldesulfurizing treatment and in which part or all of an aggregate of therecovered desulfurizing agent is separated into pieces.
 37. A methodaccording to claim 33, desulfurizing the hot metal by adding, to hotmetal, the recovered desulfurizing agent resulting from the mechanicalagitation type hot metal desulfurizing treatment and one or more sourcesselected from the group consisting of a lime source and a carbon source.38. A method according to claim 33, desulfurizing hot metal by mixingthe recovered desulfurizing agent resulting from the mechanicalagitation type hot metal desulfurizing treatment with one or moresources selected from the group consisting of a lime source and a carbonsource, and adding this mixture to hot metal.
 39. A method according toclaim 33, desulfurizing the hot metal by separately adding, to the hotmetal, the recovered desulfurizing agent resulting from the mechanicalagitation type hot metal desulfurizing treatment and one or more sourcesselected from the group consisting of a lime source and a carbon source.40. A method according to claim 37, wherein when the lime source isadded, the mixing ratio is adjusted so as to obtain a predeterminedamount of pure CaO component.
 41. A method according to claim 37,comprising a step of calculating the bulk density of the recovereddesulfurizing agent resulting from the mechanical agitation type hotmetal desulfurizing treatment, a step of calculating the amount of pureCaO component in the recovered desulfurizing agent from the calculatedbulk density, and a step of adjusting the addition ratio of the crushedrecovered desulfurizing agent to the lime source on the basis of thecalculated amount of pure CaO component in the recovered desulfurizingagent.
 42. A method according to claim 37, comprising a step ofadjusting the grain size of the carbon source to 1 mm or smaller whenthe carbon source is added.